The Little Ice Age brought colder, snowier winters to Europe, starting in about 1550. Many paintings at the time documented the climate change, including Pieter Bruegel's "Hunters in the Snow,"painted in 1565.

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Glaciers in the Alps of Europe pose a scientific mystery. They started melting rapidly back in the 1860s. In a span of about 50 years, some of the biggest glaciers had retreated more than half a mile.

But nobody could explain the glacier's rapid decline. Now, a new study from NASA's Jet Propulsion Laboratory uncovers a possible clue to why the glaciers melted before temperatures started rising: Soot from the Industrial Revolution could have heated up the ice.

Scientists trying to understand Europe's climate for the past several hundred years have turned to the glaciers in the Alps because they preserve some of the temperature and precipitation history during that time.

If you look back through the 1600s and 1700s, the glaciers were big and quite stable, says NASA's Tom Painter. That's probably because Europe was in a prolonged cold spell, known as the Little Ice Age. "And then around 1860, 1865, [the glaciers] all started to retreat to lengths that they had not in the previous few hundred years," he says.

To some historians that retreat marks the end of the Little Ice Age. But there's a problem: Europe didn't actually heat up until the 1910s or 1920s. In fact, if you go by just air temperature and precipitation, the glaciers should have advanced, not retreated. So why would the glaciers have started to melt?

"It dawned on me that industrialization was kicking off then," Painter says. "We have these visions from Charles Dickens and others of that time — the mid-1800s — of a huge amount of soot being pumped out into the atmosphere, not just in England but in France and Germany and Italy."

Painter's previous research has shown that dust blowing onto the Rocky Mountains is making the snow melt much faster there because dark snow absorbs a lot more sunlight.

Of course he couldn't sample ice from the Alps that has already melted away, but he found a record of soot from ice samples higher up in the mountains. He and his colleagues argue in the Proceedings of the National Academy of Sciences that the glaciers didn't simply melt away because the Little Ice Age petered out.

"What this tells us is that there was a human influence, very likely, reaching back all the way to what we had considered to be this natural cycle," he says.

Other scientists puzzling over the Alpine glacial melt say Painter's idea makes a lot of sense — even though it's not an ironclad case.

A team of geoscientists at Utrecht University in The Netherlands, has an automatic weather station on a glacier in Switzerland. "What we measure there is that every year the surface is darker, not due to black carbon but from dust from the surroundings," says the team's Rianne Giesen. "So I do think that dust or black carbon can make a strong contribution to the melt."

The soot idea could also explain why the glaciers in the Alps started to melt decades before glaciers upwind from Europe did. Industrial pollution from Europe wasn't blowing onto Iceland or Norway, which remained cold and frosty, says Gifford Miller at the University of Colorado.

"Maybe it gave the Alpine folks in Switzerland and the Italian Alps a bit of a head start in cultivating the highlands again," he says.

Could that be why we have Swiss milk chocolate today instead of Norwegian chocolate? "Could be," Miller says, with a chuckle.

Actually, it turns out that the Swiss did invent milk chocolate a decade or two after the glaciers started to give way to more pastureland, so it's not an entirely nutty idea.

But Miller sees a much bigger lesson from understanding how soot and dust can bring about quick changes to the world's glaciers. "Things that impact the climate system in abrupt ways are the things that people should be concerned about for the future."

And as the story of the Alps suggests, dark particles can melt the world's glaciers much more rapidly than gradual changes in the earth's temperature can.

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Transcript

STEVE INSKEEP, HOST:

And next, let's explore a mystery. The mystery takes us to the Alps of Europe - any excuse. It involves the glaciers there. Those glaciers started melting rapidly back in the 1860s. And in the span of about 50 years, some of the biggest glaciers retreated more than half a mile. Nobody could explain why. Now, a new study suggests the glaciers melted because they were covered with soot from the industrial revolution. NPR's Richard Harris reports.

RICHARD HARRIS, BYLINE: Scientists trying to understand Europe's climate for the past several hundred years have turned to the glaciers in the Alps, since they preserve some of the history of temperature and precipitation during that time. Tom Painter from NASA's Jet Propulsion Laboratory says if you look back at the 1600s and 1700s, the glaciers were quite big and stable. That observation reflects that Europe was in a prolonged cold spell known as the Little Ice Age.

TOM PAINTER: And then around 1860, 1865, they all started to retreat to lengths that they had not had in the previous few hundred years.

HARRIS: To some historians, that retreat marks the end of the Little Ice Age. But there's a problem: Europe did not actually heat up until the 1910s or 1920s. In fact, if you go just by air temperature and precipitation, the glaciers should have advanced, not retreated. So, why would the glaciers have started to melt?

PAINTER: It dawned on me, God, well, industrialization was kicking off then and we have these visions from Charles Dickens and others of that time, the mid-1800s, of a huge amount of soot being pumped out into the atmosphere, not just in England but also in France and Germany and Italy.

HARRIS: Painter's previous research has shown that dust blowing onto the Rocky Mountains is making the snow melt much faster there because dark snow absorbs a lot more sunlight. Of course, he couldn't sample ice from the Alps that had already melted away, but he found a record of soot from ice samples higher up in the mountains. He and his colleagues argue in the Proceeding of the National Academy of Sciences that the glaciers didn't simply melt away because the Little Ice Age petered out.

PAINTER: What this tells us then is that, no, in fact, there was a human influence very likely reaching back all the way to what we had considered to be this natural cycling.

HARRIS: Other scientists, puzzling over the timing of this glacial melt, said Painter's idea makes a lot of sense, even though it's not an ironclad case. Rianne Giesen is in a lab at the University of Utrecht in the Netherlands, which has been studying the alpine melt for many years.

RIANNE GIESEN: We have, for example, an automatic weather station on a glacier in Switzerland. And what we measure there is that every year the surface is darker - everything from dust from the surroundings - and we see that that really affects the melt there.

HARRIS: The soot idea could also explain why the glaciers in the Alps started to melt decades before glaciers upwind from Europe did. Gifford Miller at the University of Colorado says industrial pollution from Europe wasn't blowing into Iceland or Norway, which remained cold and frosty.

GIFFORD MILLER: Maybe it gave the Alpine folks in Switzerland and the Italian Alps a bit of a headstart in cultivating those high lands again.

HARRIS: Actually, it turns that the Swiss did invent milk chocolate a decade or two after the glaciers started to give way to more pasture land. So, it's not an entirely nutty idea. But Miller sees a much bigger lesson from understanding how soot and dust can bring about quick changes to the world's glaciers.

MILLER: Things that impact the climate system in abrupt ways are the things that people should be concerned about for the future.

HARRIS: And, as the story of the Alps suggests, dark particles can melt the world's glaciers much more rapidly than gradual changes in the Earth's temperature can.